1. Field of the Invention
The invention relates to a method for producing a coated substrate, to a panel like substrate comprising at least two layers applied by heating, and to the use of the coated substrate.
2. Description of Related Art
It is known, that inorganic, glass-based coatings endow the surfaces with particular qualities. The glass fraction of the coating, often termed glass frit or glass flux, gives the coating particular chemical and physical properties. These are, for example, chemical resistance, abrasion and scratch resistance, and thermal stability. As compared with coatings having an organic matrix, the inorganic coatings are notable for greater gloss and greater resistance toward UV irradiation. By addition of coloring pigments, antireflection additives or dulling additives, diverse possibilities are provided for differentiating and configurating the coated panes and articles produced from them in terms of aesthetics and design. The coating operation itself is usually termed glazing, enameling, decorating or printing. There are diverse, technically established methods and equipment in use for the coating. In addition to dry coating, such as by electrostatic powder application, for example, there exist numerous coating methods in which a liquid or pastelike coating material is applied. This paste may be applied to the surface of the pane by a variety of methods, as for example by screen printing, pad printing, transfer methods or else by dipping and spraying methods. The paste consists customarily of the ground glass powder, the optionally added pigments and/or additives, and the organic auxiliaries, which are removed during the baking and the flowing and leveling of the coating. For the coating of glass, glass-ceramic or ceramic panes there are a multitude of enamel compositions known.
For instance, DE 197 21 737 C1 describes a lead-free and cadmium-free glass composition for the glazing, enameling, and decorating of glasses or glass-ceramics, and also a method for producing a glass-ceramic coated with said composition.
The addition of structure-forming particles to glass-based coatings to give them special qualities is known. For instance, DE 100 16 485 A1 describes glass-based coatings on glass, glass-ceramic or metal substrates where structure-forming particles are used that have an average particle diameter in the range from 0.1 to 50 μm. The resulting, micro-rough and additionally hydrophobized surface structure endows the layer with self-cleaning properties (Lotus effect). The effect is based on a rough, non-rounded surface microstructure.
Specification EP 2 592 056 A1 as well describes layers on glass or glass-ceramic substrates that have structure-imparting inorganic particles in the coating. The particles produce elevations on the layer and hence a tactile structure. The addition of the structure-imparting particles endows the user-facing surface with a tactile function.
Articles having adhesion-reducing coatings and methods for producing them are known. For instance, US 2001/0031360 A1 describes a method for producing so-called, non-sticking, surfaces, which involves embedding diamond powder of size <50 μm into a glass frit. The glass frit acts as a binder and forms the layer, whereby the durability of articles such as cookware or leisure products is increased and the surface is endowed with “non-sticking” qualities.
It is known from GB 2 241 179 A, moreover, that polymer coatings are provided with glass beads in order to obtain surfaces having adhesion-reducing properties.
Enamel colors with adhesion-reducing properties for the coating of glass panes are known. The principle on which these adhesion-reducing properties are based is as follows. The black coatings at the edges of automobile screens are produced using glass-based enamel colors having adhesion-reducing properties. This is necessary when, during press bending, for example, the color makes contact with the pressing tool at relatively high temperatures. In the joint bending of glass panes for a correctly fitting laminate assembly as well, adhesion-reducing colors are needed if a decorated side of glass is in direct contact with another glass pane at the relatively high temperatures the operation entails. The basis for the adhesion-reducing properties of these enamel colors is the partial crystallization of the glass frit. The crystals prevent the colors from sticking at elevated temperatures. The crystallization must be closely controlled, since otherwise, if crystallization is insufficient, sticking will occur. If the crystallization of the glass frit is too great, it does not melt to a pore-free form, and, moreover, the color becomes gray. In crystallization, efforts are made, via the composition of the glass component, the temperature regime, and, optionally, through addition of nucleators, to ensure compliance with this narrow operating window. The most common crystallizing phases are bismuth-silicates, zinc silicates, and zinc borates. The narrow operating window described is economically and technically disadvantageous for this method, and the possibilities for configuration in terms of design are limited. Colors of this kind are disclosed in EP 0 895 969 A1.
The use of so-called levitation kilns allows the double-sided application of glass-based coatings, since the coating on firing has no contact with the underlay. In the critical temperature range in which the glass component melts, flows, and levels, contact is avoided by means of a gas cushion, and so there can be no sticking and no damage to the coating. In constructional terms, the ceramic underlays in this case must be gas-permeable and the quantity and temperature of the gas must be controlled in defined ways. Guidance and transport of the panes must be managed as well. It is difficult technically to integrate the operating step of thermal prestressing, which is carried out by blowing cold air onto the hot glass substrate, without a contact, since the panes have to be fixed. On account of the high operating and acquisition costs, therefore, this technique entails economic disadvantages.